In the past decade, there has been increasing interest in studying the potential of integrating voice and data traffic in both long haul and local area networks. Integration offers the capability of dynamically sharing transmission and switching facilities among different services and has the flexiblity to accommodate new services. Several alternative integrated network structures have been recently investigated. In general, they are variations of three broad switching technologies: circuit switching, packet switching, and hybrid switching.
In traditional circuit switching, a complete end-to-end circuit is established for each pair of voice and data users and dedicated for the full duration of a call. The circuit is disconnected when either party hangs up. The primary characteristics of a circuit switched network are fixed-bandwidth and low transmission delay once a connection has established. Such a circuit is inefficient in carrying bursty traffic of the type common in terminal-to-computer or computer-to-terminal communications. Moreover, on voice connections, the average actual time that speech is present on channels is less than 40 percent. This low activity is largely due to the fact that a talker speaks less than half the time during a conversation. In addition, pauses usually occur between the utterances of a speaker.
Time assignment speech interpolation (TASI) and digital speech interpolation (DSI) systems have been used in some instances to take advantage of the burstiness of voice traffic. TASI systems operate by assigning a transmission circuit to a conversation only when speech is present. During idle intervals and pauses, the transmission circuit is assigned to another conversation. DSI systems utilize, in addition to speech interpolation techniques, sophisticated speech encoding techniques to minimize the bandwidth required for the transmission of speech. The April 1982 issue to the IEEE Transactions on Communications is devoted to a discussion of these types of techniques.
Packet switching also takes advantage of the burstiness of digitized speech and data communications. User information streams are divided into segments that are combined with headers to form packets. A packet switching network routes the packets to their appropriate destinations based on addresses contained in the packet headers. In this way, the network resources are statistically shared among all users, rather than being dedicated to users on a full-time basis. An inherent characteristic of packet switched networks is large and variable end-to-end delays that may be experienced by individual packets during transport. In the case of digitized voice, large variable packet delays seriously affect the intelligibility of speech and may become intolerable. Corrective measures called time stamps are often employed to smooth the delay, but cause additional overhead. Another problem is that packets can be delivered out of order to a destination if corrective measures are not taken. Complicated transmission protocols are typically used to avoid this problem.
In hybrid switching schemes, some combination of circuit switching and packet switching is used, typically circuit switching for voice and bulk data, and packet switching is used for data. The transmission facilities are shared between both circuit switched traffic and packet switched traffic.
In addition to the three basic switching techniques, other advanced switching techniques have been studied in the literature. One such technique is called fast circuit switching. The concept is to setup and takedown a circuit through a switching node at the beginning and end of a talkspurt or data packet. A talkspurt is an interval of essentially continuous speech between pauses. Talkspurts in the range of about one second are typical. The performance of such a network is qualitatively discussed in an article by E. A. Harrington, entitled "Voice/Data Integration Using Circuit Switched Networks" and appearing in IEEE Transactions on Communications, June 1980, Vol. COM-28, No. 6. To the best of our knowledge, no implementation of a fast circuit-switched network exists at this time.
A further type of switching for integrated voice and data is briefly discussed in an article entitled "Burst Switching - An Introduction," by S. R. Amstutz and appearing at page 36 of the IEEE Communications Magazine, November 1983. The article describes a host of small switching nodes interconnected by T1 carrier links. According to the article, a fundamental difference between this system and other packet switching systems is that its T1 transmission circuit rate equals the bit-rate of a digitized voice circuit, thus eliminating the need to packetize the voice samples, and the resulting packetizing delays at the nodes. Our network is different from the burst switching network in many aspects. There are two major differences that are directly related to the aspects of the invention. We use self-routing packet switches to ease the switching of talkspurts and data packets. In addition, we use dynamic bit assignment controllers to resolve momentary overloads of transmission systems.